The long-term objective of this proposed research is to develop a simple, inexpensive point-of-care device that will purify CD4+ T cells from whole blood and perform absolute CD4+ T cell counts. This will be accomplished by adapting, combining, and testing a prototype chip-based flow-through particle counter with a separate passive microfluidic cell separation and purification technique. Toward this objective, Phase I will investigate four aims focused on establishing a proof-of-concept system using these technologies to count CD4+ T cells. Aim One will determine the feasibility of using a CD4 passive microfluidic "prep card" capable of purifying CD4+ T cells from whole blood. Aim Two will determine the feasibility of using microfabricated chips to count the isolated ceils in a disposable platform. Aim Three will determine the feasibility of using a simple electronic interface to reliably count the CD4+T cells. Aim Four will test and validate the individual CD4 counting components using commercially available normal blood specimens. Upon completion of these four aims, Phase II will focus on integrating the microfluidic separation "prep card" with the microfabricated counter chips to produce a single component, one-time use CD4 counter cartridge. This new system will then undergo further validation and optimization in preparation for field trials against existing industry gold standards for CD4 analysis. The resulting device, when fully validated and evaluated in the proposed scenarios of use, will be simple, robust, and unique in that it provides a fully quantitative CD4+ T cell analysis with an instrument costing less than $300. This is almost two orders of magnitude below the cost of established flow cytometry devices. Reagent and disposable costs will also be lower than that of any currently available device. The tool will find immediate application in all levels of the public health care system, particularly in resource-poor areas and developing countries, for monitoring persons with human immunodeficiency virus (HIV), formulating differential diagnoses, and making therapeutic decisions regarding anti-retroviral drug treatment regimes, including the highly active anti-retroviral therapy (HAART) approach. [unreadable] [unreadable] [unreadable]